Fluorescent lamps provide illumination in typical electrical devices for general lighting purposes and are more efficient than incandescent bulbs. A fluorescent lamp is a low pressure gas discharge source, in which fluorescent powders are activated by an arc energy generated by mercury plasma. When a proper voltage is applied, an arc is produced by current flowing between the electrodes through the mercury vapor, which generates some visible radiation and the resulting ultraviolet excites the phosphors to emit light. In fluorescent lamps two electrodes are hermetically sealed at each end of the bulb, which are designed to operate as either “cold” or “hot” cathodes or electrodes in glow or arc modes of discharge operation.
Cold cathode fluorescent lamps (CCFLs) are popular in backlight applications for liquid crystal displays (LCDs). Electrodes for glow or cold cathode operation may consist of closed-end metal cylinders that are typically coated on the inside with an emissive material. The current used by CCFLs is generally on the order of a few milliamperes, while the voltage drop is on the order of several hundred volts.
CCFLs have a much longer life than the hot electrode fluorescent lamps as a result of their rugged electrodes, lack of filament, and low current consumption. They start immediately, even at a cold temperature, and their life is not affected by the number of starts, and can be dimmed to very low levels of light output. However, since a large number of lamps are required for large size LCDs, balanced current sharing among lamps is required for achieving uniform backlight and long lamp life.
One means of current balancing is to drive each lamp with an independently controlled inverter, which achieves high accuracy in current sharing; however, this approach is usually complicated and expensive. Another solution is to drive all lamps with a single inverter. FIG. 1 depicts a multi-CCFL system comprising a low voltage inverter, a step-up transformer, and current balancing transformers. This technique is more cost effective. Currently there are a few current balancing transformer techniques, two of which are shown in FIGS. 2A and 2B. In these designs, the current balancing is not available under open lamp condition.